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class="sidebar-link">一、Keep-alive 是什么</a></li><li class="sidebar-sub-header"><a href="/fe/vue/keepalive.html#二、使用场景" class="sidebar-link">二、使用场景</a></li><li class="sidebar-sub-header"><a href="/fe/vue/keepalive.html#三、原理分析" class="sidebar-link">三、原理分析</a></li><li class="sidebar-sub-header"><a href="/fe/vue/keepalive.html#四、思考题-缓存后如何获取数据" class="sidebar-link">四、思考题：缓存后如何获取数据</a></li><li class="sidebar-sub-header"><a href="/fe/vue/keepalive.html#参考文献" class="sidebar-link">参考文献</a></li></ul></li><li><a href="/fe/vue/modifier.html" class="sidebar-link">面试官：Vue常用的修饰符有哪些有什么应用场景</a></li><li><a href="/fe/vue/directive.html" class="sidebar-link">面试官：你有写过自定义指令吗？自定义指令的应用场景有哪些？</a></li><li><a href="/fe/vue/filter.html" class="sidebar-link">面试官：Vue中的过滤器了解吗？过滤器的应用场景有哪些？</a></li><li><a href="/fe/vue/vnode.html" class="sidebar-link">虚拟DOM</a></li><li><a href="/fe/vue/diff.html" class="sidebar-link">Diff 算法</a></li><li><a href="/fe/vue/axios.html" class="sidebar-link">面试官：Vue项目中有封装过axios吗？主要是封装哪方面的？</a></li><li><a href="/fe/vue/axios_code.html" class="sidebar-link">面试官：你了解axios的原理吗？有看过它的源码吗？</a></li><li><a href="/fe/vue/ssr.html" class="sidebar-link">面试官：SSR解决了什么问题？有做过SSR吗？你是怎么做的？</a></li><li><a href="/fe/vue/structure.html" class="sidebar-link">面试官：说下你的vue项目的目录结构，如果是大型项目你该怎么划分结构和划分组件呢？</a></li><li><a href="/fe/vue/permission.html" class="sidebar-link">面试官：vue要做权限管理该怎么做？如果控制到按钮级别的权限怎么做？</a></li><li><a href="/fe/vue/cors.html" class="sidebar-link">面试官：Vue项目中你是如何解决跨域的呢？</a></li><li><a href="/fe/vue/404.html" class="sidebar-link">面试官：vue项目本地开发完成后部署到服务器后报404是什么原因呢？</a></li><li><a href="/fe/vue/error.html" class="sidebar-link">面试官：你是怎么处理vue项目中的错误的？</a></li><li><a href="/fe/vue/vue_router.html" class="sidebar-link">Vue Router</a></li><li><a href="/fe/vue/vue-debounce-throttle.html" class="sidebar-link">vue防抖调研</a></li></ul></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Vue3</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>React</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Webpack</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>Node.js</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>性能优化</span> <span class="arrow right"></span></p> <!----></section></li><li><section class="sidebar-group collapsable depth-0"><p class="sidebar-heading"><span>手写代码</span> <span class="arrow right"></span></p> <!----></section></li></ul> </aside> <main class="page"> <div class="theme-default-content content__default"><h1 id="面试官-说说你对keep-alive的理解是什么"><a href="#面试官-说说你对keep-alive的理解是什么" class="header-anchor">#</a> 面试官：说说你对keep-alive的理解是什么？</h1> <p><img src="https://imgconvert.csdnimg.cn/aHR0cHM6Ly9tbWJpei5xcGljLmNuL21tYml6X3BuZy9nSDMxdUY5VklpYlRaSXdpY3ZmUkR3STRiamRBVGlhVEpFZDNzamRoeTd3MDlVM0k5ZERjNUVVSUNFVk1WSVE2aDFYMjVpY1NRT3lraWFwWEpEUFM0VGJST0l3LzY0MA?x-oss-process=image/format,png" alt=""></p> <h2 id="一、keep-alive-是什么"><a href="#一、keep-alive-是什么" class="header-anchor">#</a> 一、Keep-alive 是什么</h2> <p><code>keep-alive</code>是<code>vue</code>中的内置组件，能在组件切换过程中将状态保留在内存中，防止重复渲染<code>DOM</code></p> <p><code>keep-alive</code> 包裹动态组件时，会缓存不活动的组件实例，而不是销毁它们</p> <p><code>keep-alive</code>可以设置以下<code>props</code>属性：</p> <ul><li><p><code>include</code> - 字符串或正则表达式。只有名称匹配的组件会被缓存</p></li> <li><p><code>exclude</code> - 字符串或正则表达式。任何名称匹配的组件都不会被缓存</p></li> <li><p><code>max</code> - 数字。最多可以缓存多少组件实例</p></li></ul> <p>关于<code>keep-alive</code>的基本用法：</p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token operator">&lt;</span>keep<span class="token operator">-</span>alive<span class="token operator">&gt;</span>
  <span class="token operator">&lt;</span>component <span class="token punctuation">:</span>is<span class="token operator">=</span><span class="token string">&quot;view&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>component<span class="token operator">&gt;</span>
<span class="token operator">&lt;</span><span class="token operator">/</span>keep<span class="token operator">-</span>alive<span class="token operator">&gt;</span>
</code></pre></div><p>使用<code>includes</code>和<code>exclude</code>：</p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token operator">&lt;</span>keep<span class="token operator">-</span>alive include<span class="token operator">=</span><span class="token string">&quot;a,b&quot;</span><span class="token operator">&gt;</span>
  <span class="token operator">&lt;</span>component <span class="token punctuation">:</span>is<span class="token operator">=</span><span class="token string">&quot;view&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>component<span class="token operator">&gt;</span>
<span class="token operator">&lt;</span><span class="token operator">/</span>keep<span class="token operator">-</span>alive<span class="token operator">&gt;</span>

<span class="token operator">&lt;</span><span class="token operator">!</span><span class="token operator">--</span> 正则表达式 <span class="token punctuation">(</span>使用 <span class="token string">`v-bind`</span><span class="token punctuation">)</span> <span class="token operator">--</span><span class="token operator">&gt;</span>
<span class="token operator">&lt;</span>keep<span class="token operator">-</span>alive <span class="token punctuation">:</span>include<span class="token operator">=</span><span class="token string">&quot;/a|b/&quot;</span><span class="token operator">&gt;</span>
  <span class="token operator">&lt;</span>component <span class="token punctuation">:</span>is<span class="token operator">=</span><span class="token string">&quot;view&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>component<span class="token operator">&gt;</span>
<span class="token operator">&lt;</span><span class="token operator">/</span>keep<span class="token operator">-</span>alive<span class="token operator">&gt;</span>

<span class="token operator">&lt;</span><span class="token operator">!</span><span class="token operator">--</span> 数组 <span class="token punctuation">(</span>使用 <span class="token string">`v-bind`</span><span class="token punctuation">)</span> <span class="token operator">--</span><span class="token operator">&gt;</span>
<span class="token operator">&lt;</span>keep<span class="token operator">-</span>alive <span class="token punctuation">:</span>include<span class="token operator">=</span><span class="token string">&quot;['a', 'b']&quot;</span><span class="token operator">&gt;</span>
  <span class="token operator">&lt;</span>component <span class="token punctuation">:</span>is<span class="token operator">=</span><span class="token string">&quot;view&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>component<span class="token operator">&gt;</span>
<span class="token operator">&lt;</span><span class="token operator">/</span>keep<span class="token operator">-</span>alive<span class="token operator">&gt;</span>
</code></pre></div><p>匹配首先检查组件自身的 <code>name</code> 选项，如果 <code>name</code> 选项不可用，则匹配它的局部注册名称 (父组件 <code>components</code> 选项的键值)，匿名组件不能被匹配</p> <p>设置了 keep-alive 缓存的组件，会多出两个生命周期钩子（<code>activated</code>与<code>deactivated</code>）：</p> <ul><li><p>首次进入组件时：<code>beforeRouteEnter</code> &gt; <code>beforeCreate</code> &gt; <code>created</code>&gt; <code>mounted</code> &gt; <code>activated</code> &gt; ... ... &gt; <code>beforeRouteLeave</code> &gt; <code>deactivated</code></p></li> <li><p>再次进入组件时：<code>beforeRouteEnter</code> &gt;<code>activated</code> &gt; ... ... &gt; <code>beforeRouteLeave</code> &gt; <code>deactivated</code></p></li></ul> <h2 id="二、使用场景"><a href="#二、使用场景" class="header-anchor">#</a> 二、使用场景</h2> <p>使用原则：当我们在某些场景下不需要让页面重新加载时我们可以使用<code>keepalive</code></p> <p>举个栗子:</p> <p>当我们从<code>首页</code>–&gt;<code>列表页</code>–&gt;<code>商详页</code>–&gt;<code>再返回</code>，这时候列表页应该是需要<code>keep-alive</code></p> <p>从<code>首页</code>–&gt;<code>列表页</code>–&gt;<code>商详页</code>–&gt;<code>返回到列表页(需要缓存)</code>–&gt;<code>返回到首页(需要缓存)</code>–&gt;<code>再次进入列表页(不需要缓存)</code>，这时候可以按需来控制页面的<code>keep-alive</code></p> <p>在路由中设置<code>keepAlive</code>属性判断是否需要缓存</p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token punctuation">{</span>
  path<span class="token punctuation">:</span> <span class="token string">'list'</span><span class="token punctuation">,</span>
  name<span class="token punctuation">:</span> <span class="token string">'itemList'</span><span class="token punctuation">,</span> <span class="token comment">// 列表页</span>
  component <span class="token punctuation">(</span>resolve<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token function">require</span><span class="token punctuation">(</span><span class="token punctuation">[</span><span class="token string">'@/pages/item/list'</span><span class="token punctuation">]</span><span class="token punctuation">,</span> resolve<span class="token punctuation">)</span>
 <span class="token punctuation">}</span><span class="token punctuation">,</span>
 meta<span class="token punctuation">:</span> <span class="token punctuation">{</span>
  keepAlive<span class="token punctuation">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>
  title<span class="token punctuation">:</span> <span class="token string">'列表页'</span>
 <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>使用<code>&lt;keep-alive&gt;</code></p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token operator">&lt;</span>div id<span class="token operator">=</span><span class="token string">&quot;app&quot;</span> class<span class="token operator">=</span><span class="token string">'wrapper'</span><span class="token operator">&gt;</span>
    <span class="token operator">&lt;</span>keep<span class="token operator">-</span>alive<span class="token operator">&gt;</span>
        <span class="token operator">&lt;</span><span class="token operator">!</span><span class="token operator">--</span> 需要缓存的视图组件 <span class="token operator">--</span><span class="token operator">&gt;</span> 
        <span class="token operator">&lt;</span>router<span class="token operator">-</span>view v<span class="token operator">-</span><span class="token keyword">if</span><span class="token operator">=</span><span class="token string">&quot;$route.meta.keepAlive&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>router<span class="token operator">-</span>view<span class="token operator">&gt;</span>
     <span class="token operator">&lt;</span><span class="token operator">/</span>keep<span class="token operator">-</span>alive<span class="token operator">&gt;</span>
      <span class="token operator">&lt;</span><span class="token operator">!</span><span class="token operator">--</span> 不需要缓存的视图组件 <span class="token operator">--</span><span class="token operator">&gt;</span>
     <span class="token operator">&lt;</span>router<span class="token operator">-</span>view v<span class="token operator">-</span><span class="token keyword">if</span><span class="token operator">=</span><span class="token string">&quot;!$route.meta.keepAlive&quot;</span><span class="token operator">&gt;</span><span class="token operator">&lt;</span><span class="token operator">/</span>router<span class="token operator">-</span>view<span class="token operator">&gt;</span>
<span class="token operator">&lt;</span><span class="token operator">/</span>div<span class="token operator">&gt;</span>
</code></pre></div><h2 id="三、原理分析"><a href="#三、原理分析" class="header-anchor">#</a> 三、原理分析</h2> <p><code>keep-alive</code>是<code>vue</code>中内置的一个组件</p> <p>源码位置：src/core/components/keep-alive.js</p> <div class="language-go extra-class"><pre class="language-go"><code>export <span class="token keyword">default</span> <span class="token punctuation">{</span>
  name<span class="token punctuation">:</span> <span class="token string">'keep-alive'</span><span class="token punctuation">,</span>
  abstract<span class="token punctuation">:</span> <span class="token boolean">true</span><span class="token punctuation">,</span>

  props<span class="token punctuation">:</span> <span class="token punctuation">{</span>
    include<span class="token punctuation">:</span> <span class="token punctuation">[</span>String<span class="token punctuation">,</span> RegExp<span class="token punctuation">,</span> Array<span class="token punctuation">]</span><span class="token punctuation">,</span>
    exclude<span class="token punctuation">:</span> <span class="token punctuation">[</span>String<span class="token punctuation">,</span> RegExp<span class="token punctuation">,</span> Array<span class="token punctuation">]</span><span class="token punctuation">,</span>
    max<span class="token punctuation">:</span> <span class="token punctuation">[</span>String<span class="token punctuation">,</span> Number<span class="token punctuation">]</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>

  created <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    this<span class="token punctuation">.</span>cache <span class="token operator">=</span> Object<span class="token punctuation">.</span><span class="token function">create</span><span class="token punctuation">(</span>null<span class="token punctuation">)</span>
    this<span class="token punctuation">.</span>keys <span class="token operator">=</span> <span class="token punctuation">[</span><span class="token punctuation">]</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>

  destroyed <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">const</span> key in this<span class="token punctuation">.</span>cache<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token function">pruneCacheEntry</span><span class="token punctuation">(</span>this<span class="token punctuation">.</span>cache<span class="token punctuation">,</span> key<span class="token punctuation">,</span> this<span class="token punctuation">.</span>keys<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>

  mounted <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    this<span class="token punctuation">.</span>$<span class="token function">watch</span><span class="token punctuation">(</span><span class="token string">'include'</span><span class="token punctuation">,</span> val <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
      <span class="token function">pruneCache</span><span class="token punctuation">(</span>this<span class="token punctuation">,</span> name <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token function">matches</span><span class="token punctuation">(</span>val<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span>
    this<span class="token punctuation">.</span>$<span class="token function">watch</span><span class="token punctuation">(</span><span class="token string">'exclude'</span><span class="token punctuation">,</span> val <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
      <span class="token function">pruneCache</span><span class="token punctuation">(</span>this<span class="token punctuation">,</span> name <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token operator">!</span><span class="token function">matches</span><span class="token punctuation">(</span>val<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span><span class="token punctuation">,</span>

  <span class="token function">render</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token comment">/* 获取默认插槽中的第一个组件节点 */</span>
    <span class="token keyword">const</span> slot <span class="token operator">=</span> this<span class="token punctuation">.</span>$slots<span class="token punctuation">.</span><span class="token keyword">default</span>
    <span class="token keyword">const</span> vnode <span class="token operator">=</span> <span class="token function">getFirstComponentChild</span><span class="token punctuation">(</span>slot<span class="token punctuation">)</span>
    <span class="token comment">/* 获取该组件节点的componentOptions */</span>
    <span class="token keyword">const</span> componentOptions <span class="token operator">=</span> vnode <span class="token operator">&amp;&amp;</span> vnode<span class="token punctuation">.</span>componentOptions

    <span class="token keyword">if</span> <span class="token punctuation">(</span>componentOptions<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token comment">/* 获取该组件节点的名称，优先获取组件的name字段，如果name不存在则获取组件的tag */</span>
      <span class="token keyword">const</span> name <span class="token operator">=</span> <span class="token function">getComponentName</span><span class="token punctuation">(</span>componentOptions<span class="token punctuation">)</span>

      <span class="token keyword">const</span> <span class="token punctuation">{</span> include<span class="token punctuation">,</span> exclude <span class="token punctuation">}</span> <span class="token operator">=</span> this
      <span class="token comment">/* 如果name不在inlcude中或者存在于exlude中则表示不缓存，直接返回vnode */</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span>
        <span class="token punctuation">(</span>include <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span><span class="token operator">!</span>name <span class="token operator">||</span> <span class="token operator">!</span><span class="token function">matches</span><span class="token punctuation">(</span>include<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token operator">||</span>
        <span class="token comment">// excluded</span>
        <span class="token punctuation">(</span>exclude <span class="token operator">&amp;&amp;</span> name <span class="token operator">&amp;&amp;</span> <span class="token function">matches</span><span class="token punctuation">(</span>exclude<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">)</span>
      <span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> vnode
      <span class="token punctuation">}</span>

      <span class="token keyword">const</span> <span class="token punctuation">{</span> cache<span class="token punctuation">,</span> keys <span class="token punctuation">}</span> <span class="token operator">=</span> this
      <span class="token comment">/* 获取组件的key值 */</span>
      <span class="token keyword">const</span> key <span class="token operator">=</span> vnode<span class="token punctuation">.</span>key <span class="token operator">==</span> null
        <span class="token comment">// same constructor may get registered as different local components</span>
        <span class="token comment">// so cid alone is not enough (#3269)</span>
        ? componentOptions<span class="token punctuation">.</span>Ctor<span class="token punctuation">.</span>cid <span class="token operator">+</span> <span class="token punctuation">(</span>componentOptions<span class="token punctuation">.</span>tag ? <span class="token string">`::${componentOptions.tag}`</span> <span class="token punctuation">:</span> <span class="token string">''</span><span class="token punctuation">)</span>
        <span class="token punctuation">:</span> vnode<span class="token punctuation">.</span>key
     <span class="token comment">/*  拿到key值后去this.cache对象中去寻找是否有该值，如果有则表示该组件有缓存，即命中缓存 */</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span>cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        vnode<span class="token punctuation">.</span>componentInstance <span class="token operator">=</span> cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">.</span>componentInstance
        <span class="token comment">// make current key freshest</span>
        <span class="token function">remove</span><span class="token punctuation">(</span>keys<span class="token punctuation">,</span> key<span class="token punctuation">)</span>
        keys<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span>
      <span class="token punctuation">}</span>
        <span class="token comment">/* 如果没有命中缓存，则将其设置进缓存 */</span>
        <span class="token keyword">else</span> <span class="token punctuation">{</span>
        cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">=</span> vnode
        keys<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span>
        <span class="token comment">// prune oldest entry</span>
        <span class="token comment">/* 如果配置了max并且缓存的长度超过了this.max，则从缓存中删除第一个 */</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>this<span class="token punctuation">.</span>max <span class="token operator">&amp;&amp;</span> keys<span class="token punctuation">.</span>length <span class="token operator">&gt;</span> <span class="token function">parseInt</span><span class="token punctuation">(</span>this<span class="token punctuation">.</span>max<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
          <span class="token function">pruneCacheEntry</span><span class="token punctuation">(</span>cache<span class="token punctuation">,</span> keys<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">,</span> keys<span class="token punctuation">,</span> this<span class="token punctuation">.</span>_vnode<span class="token punctuation">)</span>
        <span class="token punctuation">}</span>
      <span class="token punctuation">}</span>

      vnode<span class="token punctuation">.</span>data<span class="token punctuation">.</span>keepAlive <span class="token operator">=</span> <span class="token boolean">true</span>
    <span class="token punctuation">}</span>
    <span class="token keyword">return</span> vnode <span class="token operator">||</span> <span class="token punctuation">(</span>slot <span class="token operator">&amp;&amp;</span> slot<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>可以看到该组件没有<code>template</code>，而是用了<code>render</code>，在组件渲染的时候会自动执行<code>render</code>函数</p> <p><code>this.cache</code>是一个对象，用来存储需要缓存的组件，它将以如下形式存储：</p> <div class="language-go extra-class"><pre class="language-go"><code>this<span class="token punctuation">.</span>cache <span class="token operator">=</span> <span class="token punctuation">{</span>
    <span class="token string">'key1'</span><span class="token punctuation">:</span><span class="token string">'组件1'</span><span class="token punctuation">,</span>
    <span class="token string">'key2'</span><span class="token punctuation">:</span><span class="token string">'组件2'</span><span class="token punctuation">,</span>
    <span class="token comment">// ...</span>
<span class="token punctuation">}</span>
</code></pre></div><p>在组件销毁的时候执行<code>pruneCacheEntry</code>函数</p> <div class="language-go extra-class"><pre class="language-go"><code>function pruneCacheEntry <span class="token punctuation">(</span>
  cache<span class="token punctuation">:</span> VNodeCache<span class="token punctuation">,</span>
  key<span class="token punctuation">:</span> <span class="token builtin">string</span><span class="token punctuation">,</span>
  keys<span class="token punctuation">:</span> Array<span class="token operator">&lt;</span><span class="token builtin">string</span><span class="token operator">&gt;</span><span class="token punctuation">,</span>
  current?<span class="token punctuation">:</span> VNode
<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> cached <span class="token operator">=</span> cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span>
  <span class="token comment">/* 判断当前没有处于被渲染状态的组件，将其销毁*/</span>
  <span class="token keyword">if</span> <span class="token punctuation">(</span>cached <span class="token operator">&amp;&amp;</span> <span class="token punctuation">(</span><span class="token operator">!</span>current <span class="token operator">||</span> cached<span class="token punctuation">.</span>tag <span class="token operator">!=</span><span class="token operator">=</span> current<span class="token punctuation">.</span>tag<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    cached<span class="token punctuation">.</span>componentInstance<span class="token punctuation">.</span>$<span class="token function">destroy</span><span class="token punctuation">(</span><span class="token punctuation">)</span>
  <span class="token punctuation">}</span>
  cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">=</span> null
  <span class="token function">remove</span><span class="token punctuation">(</span>keys<span class="token punctuation">,</span> key<span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre></div><p>在<code>mounted</code>钩子函数中观测 <code>include</code> 和 <code>exclude</code> 的变化，如下：</p> <div class="language-go extra-class"><pre class="language-go"><code>mounted <span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    this<span class="token punctuation">.</span>$<span class="token function">watch</span><span class="token punctuation">(</span><span class="token string">'include'</span><span class="token punctuation">,</span> val <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
        <span class="token function">pruneCache</span><span class="token punctuation">(</span>this<span class="token punctuation">,</span> name <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token function">matches</span><span class="token punctuation">(</span>val<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span>
    this<span class="token punctuation">.</span>$<span class="token function">watch</span><span class="token punctuation">(</span><span class="token string">'exclude'</span><span class="token punctuation">,</span> val <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token punctuation">{</span>
        <span class="token function">pruneCache</span><span class="token punctuation">(</span>this<span class="token punctuation">,</span> name <span class="token operator">=</span><span class="token operator">&gt;</span> <span class="token operator">!</span><span class="token function">matches</span><span class="token punctuation">(</span>val<span class="token punctuation">,</span> name<span class="token punctuation">)</span><span class="token punctuation">)</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span>
</code></pre></div><p>如果<code>include</code> 或<code>exclude</code> 发生了变化，即表示定义需要缓存的组件的规则或者不需要缓存的组件的规则发生了变化，那么就执行<code>pruneCache</code>函数，函数如下：</p> <div class="language-go extra-class"><pre class="language-go"><code>function pruneCache <span class="token punctuation">(</span>keepAliveInstance<span class="token punctuation">,</span> filter<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token keyword">const</span> <span class="token punctuation">{</span> cache<span class="token punctuation">,</span> keys<span class="token punctuation">,</span> _vnode <span class="token punctuation">}</span> <span class="token operator">=</span> keepAliveInstance
  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token keyword">const</span> key in cache<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">const</span> cachedNode <span class="token operator">=</span> cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>cachedNode<span class="token punctuation">)</span> <span class="token punctuation">{</span>
      <span class="token keyword">const</span> name <span class="token operator">=</span> <span class="token function">getComponentName</span><span class="token punctuation">(</span>cachedNode<span class="token punctuation">.</span>componentOptions<span class="token punctuation">)</span>
      <span class="token keyword">if</span> <span class="token punctuation">(</span>name <span class="token operator">&amp;&amp;</span> <span class="token operator">!</span><span class="token function">filter</span><span class="token punctuation">(</span>name<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">pruneCacheEntry</span><span class="token punctuation">(</span>cache<span class="token punctuation">,</span> key<span class="token punctuation">,</span> keys<span class="token punctuation">,</span> _vnode<span class="token punctuation">)</span>
      <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>
  <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>在该函数内对<code>this.cache</code>对象进行遍历，取出每一项的<code>name</code>值，用其与新的缓存规则进行匹配，如果匹配不上，则表示在新的缓存规则下该组件已经不需要被缓存，则调用<code>pruneCacheEntry</code>函数将其从<code>this.cache</code>对象剔除即可</p> <p>关于<code>keep-alive</code>的最强大缓存功能是在<code>render</code>函数中实现</p> <p>首先获取组件的<code>key</code>值：</p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token keyword">const</span> key <span class="token operator">=</span> vnode<span class="token punctuation">.</span>key <span class="token operator">==</span> null? 
componentOptions<span class="token punctuation">.</span>Ctor<span class="token punctuation">.</span>cid <span class="token operator">+</span> <span class="token punctuation">(</span>componentOptions<span class="token punctuation">.</span>tag ? <span class="token string">`::${componentOptions.tag}`</span> <span class="token punctuation">:</span> <span class="token string">''</span><span class="token punctuation">)</span>
<span class="token punctuation">:</span> vnode<span class="token punctuation">.</span>key
</code></pre></div><p>拿到<code>key</code>值后去<code>this.cache</code>对象中去寻找是否有该值，如果有则表示该组件有缓存，即命中缓存，如下：</p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token comment">/* 如果命中缓存，则直接从缓存中拿 vnode 的组件实例 */</span>
<span class="token keyword">if</span> <span class="token punctuation">(</span>cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    vnode<span class="token punctuation">.</span>componentInstance <span class="token operator">=</span> cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span><span class="token punctuation">.</span>componentInstance
    <span class="token comment">/* 调整该组件key的顺序，将其从原来的地方删掉并重新放在最后一个 */</span>
    <span class="token function">remove</span><span class="token punctuation">(</span>keys<span class="token punctuation">,</span> key<span class="token punctuation">)</span>
    keys<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span>
<span class="token punctuation">}</span> 
</code></pre></div><p>直接从缓存中拿 <code>vnode</code> 的组件实例，此时重新调整该组件<code>key</code>的顺序，将其从原来的地方删掉并重新放在<code>this.keys</code>中最后一个</p> <p><code>this.cache</code>对象中没有该<code>key</code>值的情况，如下：</p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token comment">/* 如果没有命中缓存，则将其设置进缓存 */</span>
<span class="token keyword">else</span> <span class="token punctuation">{</span>
    cache<span class="token punctuation">[</span>key<span class="token punctuation">]</span> <span class="token operator">=</span> vnode
    keys<span class="token punctuation">.</span><span class="token function">push</span><span class="token punctuation">(</span>key<span class="token punctuation">)</span>
    <span class="token comment">/* 如果配置了max并且缓存的长度超过了this.max，则从缓存中删除第一个 */</span>
    <span class="token keyword">if</span> <span class="token punctuation">(</span>this<span class="token punctuation">.</span>max <span class="token operator">&amp;&amp;</span> keys<span class="token punctuation">.</span>length <span class="token operator">&gt;</span> <span class="token function">parseInt</span><span class="token punctuation">(</span>this<span class="token punctuation">.</span>max<span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">pruneCacheEntry</span><span class="token punctuation">(</span>cache<span class="token punctuation">,</span> keys<span class="token punctuation">[</span><span class="token number">0</span><span class="token punctuation">]</span><span class="token punctuation">,</span> keys<span class="token punctuation">,</span> this<span class="token punctuation">.</span>_vnode<span class="token punctuation">)</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>表明该组件还没有被缓存过，则以该组件的<code>key</code>为键，组件<code>vnode</code>为值，将其存入<code>this.cache</code>中，并且把<code>key</code>存入<code>this.keys</code>中</p> <p>此时再判断<code>this.keys</code>中缓存组件的数量是否超过了设置的最大缓存数量值<code>this.max</code>，如果超过了，则把第一个缓存组件删掉</p> <h2 id="四、思考题-缓存后如何获取数据"><a href="#四、思考题-缓存后如何获取数据" class="header-anchor">#</a> 四、思考题：缓存后如何获取数据</h2> <p>解决方案可以有以下两种：</p> <ul><li><p>beforeRouteEnter</p></li> <li><p>actived</p></li></ul> <h3 id="beforerouteenter"><a href="#beforerouteenter" class="header-anchor">#</a> beforeRouteEnter</h3> <p>每次组件渲染的时候，都会执行<code>beforeRouteEnter</code></p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token function">beforeRouteEnter</span><span class="token punctuation">(</span>to<span class="token punctuation">,</span> from<span class="token punctuation">,</span> next<span class="token punctuation">)</span><span class="token punctuation">{</span>
    <span class="token function">next</span><span class="token punctuation">(</span>vm<span class="token operator">=</span><span class="token operator">&gt;</span><span class="token punctuation">{</span>
        console<span class="token punctuation">.</span><span class="token function">log</span><span class="token punctuation">(</span>vm<span class="token punctuation">)</span>
        <span class="token comment">// 每次进入路由执行</span>
        vm<span class="token punctuation">.</span><span class="token function">getData</span><span class="token punctuation">(</span><span class="token punctuation">)</span>  <span class="token comment">// 获取数据</span>
    <span class="token punctuation">}</span><span class="token punctuation">)</span>
<span class="token punctuation">}</span><span class="token punctuation">,</span>
</code></pre></div><h3 id="actived"><a href="#actived" class="header-anchor">#</a> actived</h3> <p>在<code>keep-alive</code>缓存的组件被激活的时候，都会执行<code>actived</code>钩子</p> <div class="language-go extra-class"><pre class="language-go"><code><span class="token function">activated</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
   this<span class="token punctuation">.</span><span class="token function">getData</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token comment">// 获取数据</span>
<span class="token punctuation">}</span><span class="token punctuation">,</span>
</code></pre></div><p>注意：服务器端渲染期间<code>avtived</code>不被调用</p> <h2 id="参考文献"><a href="#参考文献" class="header-anchor">#</a> 参考文献</h2> <ul><li>https://www.cnblogs.com/dhui/p/13589401.html</li> <li>https://www.cnblogs.com/wangjiachen666/p/11497200.html</li> <li>https://vue3js.cn/docs/zh</li></ul></div> <footer class="page-edit"><!----> <div class="last-updated"><span class="prefix">更新时间:</span> <span class="time">12/14/2021, 10:12:35 AM</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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